/Engine/Source/Runtime/Core/Public/Misc/CoreDelegates.h
Generally, events triggering UserMusicInterruptDelegate or AudioMuteDelegate happen only when a change occurs. When a system comes online needing the current audio state but the event has already been broadcast, calling ApplicationRequestAudioState will force the UserMusicInterruptDelegate and AudioMuteDelegate to be called again if the low-level application layer supports it. Currently, this is available only on iOS.
static [TMulticastDelegate](API\Runtime\Core\Delegates\TMulticastDelegate)< void()> ApplicationRequestAudioState;